Beta-adrenergic receptors on intact heart cells will be characterized by binding (I125)-iodopindolol to embryonic chick heart ventricles. The number of beta-adrenergic receptors and their binding dissociation constants (KD) will be determined from equilibrium and kinetic binding studies. Values of KD for other beta-adrenergic agonists and antagonists will be calculated based on their ability to compete with these radioligands for specific binding sites. Dissociation binding constants will be compared to agonist activation constants (KAct) and antagonist inhibition constants (KI) for physiologic responses and cyclic AMP synthesis in intact cultured myocardial cells. The ratios KD/KAct will be used as an index of the coupling efficiency of beta-adrenergic receptors to adenylate cyclase. Interventions known to alter the physiologic responses of cardiac muscle to beta-adrenergic agents will be examined to determine how they affect beta-adrenergic receptor number, affinity (for agonists and antagonists) and coupling in intact cells. These interventions include hormones (acetylcholine, adenosine and thyroxine), and agonist-induced desensitization. The influence of noncompetitive antagonists, which functionally reduce the number of beta-adrenergic receptors, on agonist potency, efficacy (maximum response), and coupling efficiency in intact heart cells will be determined.